Abstract: Reflective mode coupling refractive index gratings are disclosed. The gratings can couple light of wavelength .lambda..sub.i in a fundamental spatial mode of the waveguide (e.g., LP.sub.01 to a reflected higher order spatial mode (e.g., LP.sub.11), substantially without reflection of any light of wavelength .lambda..sub.j .noteq..lambda..sub.i in a spectral range .DELTA..lambda..ltoreq.0.01 .lambda..sub.i. The mode coupling gratings (MCGs) can find a variety of uses in optical waveguide systems. Exemplarily, an MCG can serve as a wavelength-dependent loss element with abrupt (e.g., .about.1 nm) spectral dependence. However, a chirped grating with or without strength modulation can yield an MCG having relatively wide spectral dependence, including variable loss over a relatively wide (e.g., .about.10 nm) spectral range. Both types of MCGs are advantageously used in, for instance, optical waveguide amplifiers.